An electro-optic modulation device using electro-optic crystal is used as an electro-optic modulator which modulates the phase of light passed through the crystal according to the magnitude of the electric field generated between electrodes, or as an electric-field sensor for conversely detecting a phase change of light passed through the crystal and thereby detecting the electric field between the electrodes or an electric signal.
For example, in the electric-field sensor, an optical beam is incident on electro-optic crystal with AC electric field applied thereto and light emitted from the electro-optic crystal is separated into S-polarized light and P-polarized light by a polarizing-beam splitter (hereafter referred to as PBS). The polarized lights are detected respectively and independently by two photodetectors (hereafter referred to as PD), and a difference between intensities of the S-polarized light and the P-polarized light is detected by the PD and a differential amplifier.
FIG. 1 is a diagram showing operation of a conventional electric-field sensor.
An optical beam emitted from a light source 101 is transmitted through a phase compensator 105 and electro-optic crystal 107, and then incident on a PBS 109. The polarization state of the optical beam 103 is adjusted by the phase compensator 105 so as to become circularly polarized light immediately before incidence on the PBS 109. An electric field depending on a signal 115 to be measured is applied to the electro-optic crystal 107 via a signal electrode 111 and a ground electrode 113. The optical beam 103 is subjected to polarization modulation in the electro-optic crystal 107 according to the electric field. The polarized modulated light is separated into an S-polarized component and a P-polarized component by the PBS 109. At this time, each polarized component has already been converted to intensity modulated light. The intensity modulated S-polarized component and P-polarized light change in phases opposite to each other. Accordingly, by receiving light in PDs 117 and 119 and conducting differential signal detection in a differential amplifier 121, therefore, it becomes possible to obtain an output signal 122 having a higher signal-to-noise ratio (see, for example, Japanese Patent Application Laid-Open Nos. 2003-98205, 2003-98204 and 2000-171488).
The electro-optic modulation device using electro-optic crystal begins to be applied to communication between wearable computers using a living body as a signal path. In other words, by inducing electric field in a receiver in a wearable computer of communication destination via the living body and detecting the electric field by using an electro-optic technique, communication that does not depend upon the positional relation between the ground of the wearable computer and the earth ground to the utmost, that is communication with a wearable computer that is in an arbitrary position on the living body, can be certainly implemented.
FIGS. 2A to 2C are diagrams to explain a process for fabricating an electro-optic modulation device by using electro-optic crystal.
An electro-optic modulation device including electro-optic crystal and a pair of electrodes is formed by working thinly electro-optic crystal 107a of a raw material as shown in FIG. 2A to form a thin electro-optic crystal 107 as shown in FIG. 2B, and forming a pair of electrodes 111 and 113 on a pair of opposite side faces of the electro-optic crystal 107 worked to become thin. By the way, an electro-optic crystal 101a thus worked to become thin has a thickness d of approximately 0.1 mm.
As communication using a living body as the transmission path, several modes are conceivable. As representative modes, two modes such as communication between an installation type terminal and a portable terminal, and communication between portable terminals are conceivable.
In the communication between an installation type terminal and a portable terminal, communication can be conducted in a comparatively stable state since the installation type is connected to the earth ground. On the other hand, in the communication between portable terminals, communication is conducted in an extremely unstable state since neither of the terminals is grounded. Furthermore, battery drive is conducted typically and low power consumption is demanded. Therefore, conditions imposed on a receiver to establish communication in such a state are high sensitivity and flatness in the frequency characteristics of the sensitivity.
First studying the sensitivity, a phase change (Δφ) given to light by the electro-optic modulation device is given by the following expression.Δφ=α·(V/d)·L
Here, α is a constant depending upon the kind of the electro-optic crystal and the structure of the device, V is a voltage applied to the electrodes, d is a distance between the electrodes, and L is a length of the electro-optic modulation device. As represented by the expression, a greater phase change can be given to light as d becomes small and L becomes large. In other words, the modulation efficiency becomes high as an electro-optic modulator, and the sensitivity is improved as an electric-field sensor.
In order to shorten the distance between the pair of electrodes to the utmost, it is necessary to work the electro-optic crystal to make it thin. In the conventional technique, however, there is a problem that it is extremely difficult to generate a device using thin crystal having a thickness of mm order or less and the electro-optic crystal becomes apt to break.
It is desirable to apply antireflection coating to an end face of the electro-optic crystal on which light is incident. However, there is a problem that it becomes difficult to apply the antireflection coating if the electro-optic crystal is made thin.
From a different point of view, the sensitivity of the electro-optic modulation device serving as an electric-field sensor can be improved by lengthening the length of the electro-optic crystal in a light passage direction as described above. If the electro-optic modulation device is provided with a specific structure in order to increase the intensity by making the electro-optic crystal thin, then a phenomenon that light does not emit from the end face of the electro-optic crystal and light leaks in a side face direction because of light diffraction as the length is made longer is caused, resulting in a lowered modulation efficiency or a lowered sensitivity.
As for the flatness of the frequency characteristics which is the second demand, the following fact poses a problem. That is, in the electro-optic crystal with electric field applied thereto, the birefringence index of the crystal with respect to light is changed by deformation of the electron cloud and crystal lattice. The degree of the deformation of the electron cloud does not depend on the frequency of the applied electric field, however, the degree of the deformation of the crystal lattice depends upon the frequency. In the band of kHz to MHz order, therefore, the frequency characteristics of the sensitivity of the electro-optic crystal do not become flat in general. The reason why the frequency characteristics of the electro-optic crystal do not become flat is specifically that the eigenmode of the elastic vibration is caused depending upon the size and shape of the crystal.
In view of these problems, the present invention has been achieved. An object of the present invention is to provide an electro-optic modulation device capable of improving the modulation efficiency and sensitivity.
In particular, an object of the present invention is to provide an electro-optic modulation device capable of improving the modulation efficiency and sensitivity without hampering the strength of the device and causing a leak of light due to diffraction even when the gap between the pair of electrodes is made narrow.
Further, more specifically, an object of the invention is to provide an electro-optic modulation device which has flatness in the frequency characteristic.